Apple DOS was the family of disk operating systems for the Apple II series of microcomputers from late 1978 through early 1983. Apple DOS had three major releases: DOS 3.1, DOS 3.2, and DOS 3.3; each one of these three releases was followed by a second, minor "bug-fix" release, but only in the case of Apple DOS 3.2 did that minor release receive its own version number, Apple DOS 3.2.1. The best-known and most-used version was Apple DOS 3.3 in the 1980 and 1983 releases. Prior to the release of Apple DOS 3.1, Apple users had to rely on audio cassette tapes for data storage and retrieval, but that method was notoriously slow, inconvenient, and unreliable.
When Apple Computer introduced the Apple II in April 1977, the new computer had no disk drive or disk operating system (DOS). Although Apple co-founder Steve Wozniak designed the Disk II controller late that year, and believed that he could have written a DOS, fellow founder Steve Jobs decided to outsource the task. The company considered using Digital Research's CP/M, but Wozniak sought an operating system that was easier to use. On 10 April 1978 Apple signed a $13,000 contract with Shepardson Microsystems to write a DOS and deliver it within 35 days. Apple provided detailed specifications, and early Apple employee Randy Wigginton worked closely with Shepardson's Paul Laughton as the latter wrote the operating system with punched cards and a minicomputer.
There was no Apple DOS 1 or 2, per se. Versions 0.1 through 2.8 were serially enumerated revisions during development, which might as well have been called builds 1 through 28. Apple DOS 3.0, a renamed issue of version 2.8, was never publicly released due to bugs. To the dismay of many programmers, Apple published no official documentation until release 3.2.
Apple DOS 3.1 was publicly released in June 1978, slightly less than one year after the Apple II was introduced, becoming the first disk-based operating system for any Apple computer. A bug-fix release came later, addressing a problem with its MASTER CREATE utility, which was used to create Apple DOS master disks: The built-in INIT command created disks that could be booted only on machines with at least the same amount of memory as the one that had created them. MASTER CREATE included a self-relocating version of DOS that would boot on Apples with any memory configuration.
Apple DOS 3.2 was released in 1979 to reflect major changes in computer booting methods that were built into the successor of the Apple II, the Apple II Plus. Instead of the original Integer BASIC, the Apple II Plus firmware included the newer Applesoft II floating point BASIC. The new firmware also had an auto-start feature which would automatically find a disk controller and boot from it when the system was powered up—earning it the name Autostart ROM.
Apple DOS 3.3 was released in 1980. It improved various functions of release 3.2, while also allowing for large gains in available floppy disk storage; the newer P5A/P6A PROMs in the disk controller could read and write data at a higher density, so that instead of 13 sectors (3.25 kB), 16 sectors (4 kB) of data could be stored per disk track, increasing the capacity from 113.75 kB to 140 kB per disk side — 16 kB of which was used by filesystem overhead and a copy of DOS, on a DOS 3.3-formatted disk, leaving 124 kB for user programs and data. DOS 3.3 was, however, not backwards compatible; it could not read or write DOS 3.2 disks. To address this problem, Apple Computer released a utility called "MUFFIN" to migrate Apple DOS 3.2 files and programs to version 3.3 disks. Apple never offered a utility to copy the other way. To migrate Apple DOS 3.3 files back to version 3.2 disks, someone wrote a "NIFFUM" utility. There were also commercial utilities (such as Copy II Plus) that could copy files from and to either format (and eventually ProDOS as well). Release 3.3 also improved the ability to switch between Integer BASIC and Applesoft BASIC, if the computer had a language card (RAM expansion) or firmware card.